Formation mechanism of starch nanocrystals from waxy rice starch and their separation using differential centrifugation

[Objective]In order to explore the formation mechanism of starch nanocrystals(SNC)during the whole hydrolysis process and proposed an interesting point on SNC separation by using differential centrifugation for collecting a suitable SNC product.[Method]Waxy rice starch was hydrolyzed by 3.16 M sulfuric acid solution preparing 15%wt suspension.The samples were collected at 1,2,3,5 and 7 days by successive centrifugations and were freeze-dried.The hydrolysis kinetics,micromorphology,particle size distribution,zeta potential,molecular weight,chain length distribution,crystalline and thermal properties were evaluated.And differential centrifugation was employed in SNC separation.[Result]The hydrolysis process for waxy rice starch was divided into rapid hydrolysis stage in the initial 2 days and then slow hydrolysis stage.At hydrolysis of 2 days,the SNC product showed a nano-size of average diameter of 244 nm.With continuous hydrolysis,the crystalline degree and size and melting peak temperature and enthalpy increased.Meanwhile,the proportion of DP 6-12 chain declined,and the main proportion of DP 13-24 chains and molecular weight had insignificant change.Slow hydrolysis stage could be regarded as surface modification and gradual release of SNC.Furthermore,differential centrifugation displayed a good separation effect on the SNC product at hydrolysis of 5 days,because the product had small size and high charge density.[Conclusion]Waxy rice starch was used in SNC produce consisted of two steps,i.e.,rapid hydrolysis stage within the initial 2 days and slow hydrolysis stage 2 days later.Slow hydrolysis stage was associated with the release and surface modification of SNC.The average size was reached at nanoscale after hydrolysis of 2 days.Differential centrifugation exhibited a good separation effect for SNC with small particle size and high charge density.